In the heavy truck and trailer field, the LA system is much less well known as an alternate to the usual A dolly system (A train) than is the B dolly (B train) system. (Lately these B dolly systems seem to be referred to as `C dolly` systems, but the older terminology is used here as it is used in the UMTRI reference cited below.) There may be a few thousand B dollys in use in North America, but only about 250 LA dollys are in use here as of now. Nearly 150 of these are in gasoline tanker delivery service in the state of Michigan and another 100 or so are used in tanker service in the Canadian Province of Sasketchewan.
For a description of the geometry and the performance of LA systems in general, refer to the U.S. Pat. No. 4,262,920 of Mettatal, deemed the most pertinate prior art. More important in terms of mathematical analysis and performance details is the UMTRI report: Improving the dynamic performance of multi-trailer vehicles: A study of innovative dollies, C. B. Winkler, et. al.
The UMTRI report also discusses the B dolly system which is of interest here for performance and stress level comparisons. Some possible hardware matching between B dolly systems and the present invention are discussed below.
While the UMTRI report does show the B dolly system as the most stable of the alternate dolly systems, it also explains that B dollys are very expensive in dollar cost. Also the B dolly system is very heavy, complex, and introduces large mechanical stresses at the rear of the leading trailer.
Present LA systems offer roughly half of the overall stability improvement over A trains relative to B trains. The stress level introduced into the rear of the leading trailer by a LA dolly is about half that from a B dolly. The present practice LA systems have weight and dollar penalties about a third greater than A trains compared to B trains.
The various cost, weight stress and complexity factors of present LA systems should make them more useful in the continent's doubles and triples fleets than is seen at present. The time consuming and difficult hook up process of the present LA systems is the significant complicating factor of an otherwise simple and relatively inexpensive system. In fact, the present practice LA systems are viewed as useful only with `married` trailers that are seldom, if ever, disconnected. At present LA is seen only on tanker type service.
The conventional A dolly is of interest here not only for baseline comparison when it is used in an A train, but also because it is part of the present practice type LA systems. Some A dollys are detachable from the second trailer and have fifth wheels, (converter dollys) and others are non detachable (turntable dollys). All present A dollys have a tongue that serves both to steer the dolly and to pull the dolly and second trailer.
Present practice LA systems could be conceptualized as `a regular A dolly with an additional stiffening member between the bodies of the two trailers`. This stiffening member is articulated for some movement in the pitch axes at two places: the front of the second trailer body and the rear of the first trailer body. The stiffening member is also articulated in yaw at the connection to the front of the second trailer. In function and structure the stiffening member is designed to keep the front portion of the second trailer in line with an extended centerline of the leading trailer. In the present practice LA systems the stiffening member must be telescoping to avoid binding in turns.
Present practice LA systems could be summed up as a telescoping stiffening member used as an accessory to the regular A dolly system. This keeps the front center of the second trailer in line with an imagianary extension of the longitudinal centerline of the leading trailer in turns.
All LA systems including the present invention offer a fixed amount of steering of the dolly for any given angle of articulation of the leading trailer body relative to the second trailer body. Hence the term `linked articulation`. Some design or experimental effort is required to position the longitudinal location of the articulation points so that side slipping forces are not generated at the tires of the dolly in turning maneuvers.
LA systems including the present invention can be backed in the field as well as B dolly equipped doubles. A train doubles cannot be backed up for any reasonable distance.
A Canadian patent application has apparently been filed recently that modifies the present type LA system to couple the leading and second trailers in roll. The connection of the stiffening member to the front of the rear trailer body will now be coupled in roll while retaining articulation in yaw and pitch. Also the telescoping portion of the stiffening member will have to transmit roll forces between the two trailer bodies.